The present invention relates to a process for producing a diaphragm of vitreous hard carbonaceous material for use in an acoustic device. More particularly, the invention relates to a process for producing a diaphragm of vitreous hard carbonaceous material having high hardness, high strength and high elasticity as compared with a conventional diaphragm material used as a speaker and a microphone, the diaphragm undergoing less deformation when subjected to an external force due to excellent rigidity of the diaphragm. The diaphragm also exhibits small distortion of sound, wide sound range reproduction and distinct sound quality. In addition, the internal structure of the diaphragm and the voice coil bobbin provide high rigidity of the entire vibration system so that driving forces generated in a voice coil are transmitted to the diaphragm without loss since energy is not lost at a bonded portion having decreased rigidity. The diaphragm provides excellent responsiveness to input signals and is adapted for digital audio use.
It is generally desired that a diaphragm for a speaker and a voice coil bobbin satisfy the following conditions:
(1) small density, PA0 (2) large Young's modulus, PA0 (3) large propagating velocity of sounds, PA0 (4) adequately large internal loss of vibration, PA0 (5) stability against variation in atmospheric conditions, PA0 (6) resistance to deformation and change of properties, and PA0 (7) simple and inexpensive producing process.
More specifically the material for the diaphragm is required to have high fidelity over a broad frequency band. To efficiently and distinctly produce sound quality, the material should have high rigidity and no distortion such as creep against external stress To further increase the sound velocity (calculated from the equation of EQU V=(E/.rho.).sup.1/2
where V is sound velocity, E is Young's modulus, .rho.0 is density), the material is required to have small density and high Young's modulus.
In addition to the above-mentioned conditions, in the case of a voice coil bobbin, the material must have resistance to Joule heat generated due to voice current flowing in a voice coil.
The conventional materials for the diaphragm and voice coil bobbin use paper (pulp), plastic, aluminum, titanium, magnesium, beryllium, or boron as basic materials, and further contain glass fiber, carbon fiber, or processed metal alloy, metal nitride, metal carbide, or metal boride mixed with the basic material. However, paper, plastic and their composite materials have small Young's modulus and small density. Thus, the sound velocities of these materials are low. Vibration division occurs in a specific mode and the frequency characteristics in the high frequency band of the materials are particularly low, resulting in difficulty in producing distinct sound quality. In addition, these materials are affected by external environments such as temperature and moisture, causing deterioration in the quality and aging fatigue.
On the other hand, when metal plates of aluminum, magnesium or titanium are employed for the diaphragm and voice coil bobbin, the sound velocities of the materials are high, but the materials exhibit sharp resonance in high frequency bands and small internal loss of vibration or aging fatigue such as creep occurs in the materials. The use of materials such as boron, beryllium, their nitrides, carbide or boride for the diaphragm and voice coil bobbin provides excellent physical properties. The use of such materials as the diaphragms in tweeters extends the range of audible frequency bands which can be reproduced, so that natural sound quality can be produced without transient phenomena caused by signals in the audible band. However, these materials are very expensive, and are difficult to machine. The conventional process for producing the diaphragm by rolling and press molding is not practical and should employ a depositing method such as chemical vapor deposition or physical vapor deposition. These processes are expensive and it is difficult to produce speakers of large size.
In addition to the above-described defects, even if materials having excellent physical properties are used for a diaphragm, since the materials for the conventional voice coil bobbin are mainly paper (pulp), such as kraft paper, the rigidity of the entire vibration system resultantly decreases, the rigidity of the entire vibration system also decreases due to the presence of a bonding layer for bonding the diaphragm to the voice coil bobbin, and the materials have drawbacks in that an energy loss occurs at the bonding layer when transmitting the driving force generated in the voice coil to the diaphragm.